Type 1 Unique RectangleA Type 1 Unique Rectangle occurs when one of the four cells in which a non-unique rectangle may appear contains one or more candidates. To prevent the occurrence of the non-unique rectangle, you can remain only those additional candidates in this cell.
In the example below, if the digits 3 and 5 were excluded from the R2C4 cless, the cells R2C2, R2C4, R3C2 and R3C4 would form a non-unique rectangle. To prevent this, the digits 7 and 9 are eliminated from the R2C4 cell. Thanks to this elimination, the digit 7 in the upper middle box can only be placed in the R3C4 cell.